Bioleaching of heavy metals in municipal sludge\|optimum parameters and reaction mechanism

The selection of optimum inoculum for bioleaching of sludge was carried out, and the mechanisms of transformation of heavy metals and biological acidification during bioleaching were also investigated. The results showed that higher bioleaching efficiencies of 96\^5%, 41\^4% and 82\^9% for Cu~+/Cu~(2+), Pb~(2+) and Zn~(2+) respectively from sludge with 2% or more inoculum could be achieved during 4～6?d. For Cu~+/Cu~(2+), bioleaching was controlled by the transformation of sulfide to exchangeable forms. Pb~(2+) was leached due to transformation of carbonate precipitated and organically bound by the combination of direct and indirect mechanism. The indirect mechanism played predominant role in the bioleaching of Zn~(2+) for most of the carbonate precipitated and organically bound forms transformed to exchangeable ones. The pH of sludge was about 1.6 at the end of the process under the acidification role of Thiobacillus. In addition, the variation of NH~+_4-N and NO~-_3-N in leachate was discussed to clarify the hydrolysis of sludge cells during bioleaching.